1. Field of the Invention
The present invention relates to a gain control amplification circuit and, more particularly, to a wide-range gain control amplification circuit which has a plurality of stages of gain control amplifiers and can control the transmission output to a predetermined output value, and a terminal equipment having the circuit.
2. Description of the Prior Art
Recent radio communication requires high-speed communication and effective use of frequency resources. For these purposes, use of CDMA (Code Division Multiple Access) has been examined. In the CDMA, data are multiplexed and transmitted to effectively use the available frequency range.
The CDMA also requires control of the transmission power in a wide range to expand the communication area.
FIG. 1 shows a conventional gain amplification circuit. In the conventional gain amplification circuit, two gain amplifiers are arranged at the former and latter stages before an antenna 50 to widen the transmission range. More specifically, a gain control amplifier (IF-GCA) 20 capable of gain control is provided at the former stage, and a gain amplifier 25 having a fixed gain is arranged at the latter stage. The gain of the gain control amplifier 20 at the former stage is controlled by a control means and further amplified by the gain amplifier 25 at the latter stage, thereby obtaining a desired transmission power.
That is, a voice (analog) signal input from a microphone 6 is converted into a digital signal by a codec 8, encoded by a DSP 10 on the basis of an algorithm such as GSM-AMR as a codec scheme, and the transmission frame format, and input to an HPSK circuit 14. The transmission data HPSK-modulated by the HPSK circuit 14 is converted from the digital signal to an analog baseband signal by a D/A converter 16.
The converted analog baseband signal is modulated into the IF band by an MOD 18 on the basis of an Lo2 from a PLL 42. The IF-modulated signal is amplified to a predetermined level by the IF-GCA 20 controlled by a linear interpolation circuit 34 at the former stage. The IF-modulated signal amplified here is converted into a transmission frequency (RF-modulated signal) by an up converter 22 on the basis of an Lo1 from the PLL 42. The RF-modulated signal is amplified by the fixed gain of the gain amplifier 25 and power amplifier 26, supplied to the antenna 50, and transmitted into air.
In a telephone of CDMA scheme, since the reception intensity at the base state is constant, the transmission output from the terminal equipment is increased/decreased. When the communication area expands, the amplification width of the transmission output value required by the terminal equipment must be made large. However, the amplification width of the gain control amplifier is limited. A gain control amplifier of a single stage has a narrow range and therefore cannot cope with control based on the amplification request from a base station in a wide area.
That is when the gain of the gain amplifier 25 installed at the latter stage is increased, the gain amplifier cannot cope with a small transmission output when, e.g., the device is near the base station. Conversely, when the gain of the gain amplifier 25 is made small, the gain control amplifier cannot amplify the power to a necessary transmission output when, e.g., the device is far from the base station.
In addition, a gain range that provides the best characteristics in the characteristics of each GCA cannot be designated.
Furthermore, when the transmission output is gradually decreased, to improve the SNR, it is advantageous to decrease the gain of the GCA at the latter stage and then decrease the gain of the GCA at the former stage. However, the gain control amplifiers cannot be individually controlled.